Brake control apparatus



Julie 14, 1960 F. ABELL BRAKE CONTROL'APPARATUS Filed Jan. 3G, 1956 Jz'cex.

.Men/vz A8511.,

INVENTR.

nited States Patent Ohce 2,940,797 Patented June 14, 1915i?V BRAKE CONTROL APPARATUS Frank Abell, 631 Augusta St., Inglewood, Calif.

Filed Jan. 30, 1956, Sel'. N0. 562,190

Claims. (Cl. 303-24) The present invention relates generally to iluid pressure actuated brake systems for vehicles, such as automobiles and trucks; and more especially to means for regulating the rate or degree of application of the brakes in response to variable factors, including the speed of the vehicle at the time of application of the brakes and the rate of deceleration produced by the brakes.

The need is widely recognized -for brakes which are adequate to stop the vehicle as quickly as possible in order to produce the maximum degree of safety; but the fastest stops are not always the safest. The increasingly high speed of traine, particularly within metropolitan areas, has imposed an increasingly severe task upon brake systems. This in turn has led to various improvements and additions to brake systems for producing more eective brakes. Along this line is the change vfrom two wheel to four wheel brakes, from mechanical to hydraulic or pneumatic systems, and the addition of various power booster devices which multiply the braking elort that can be produced by the vehicle operator but at the same time tend to reduce the degree of control which the'driver has over the brakes.

As a consequence of these improvements in design, brake systems now on automotive vehicles are capable of producing high rates of deceleration under emergency conditions. Under these conditions, passengers inthe vehicles can be literally thrown around inside the vehicle with suiiicient force that they may be injured even though there is no collision between two vehicles. The rapid cle-V crease in speed now made possible is a prolific source of injuries to passengers; and it is a source Vof considerable concern to companies which operate passenger buses, especially buses which operate incongested traic in metropolitan areas. Y In an etiort to eliminate this hazard of personal injury, devices have been suggested which limit the rate of application of the brakes. Of course the obvious danger is that the rate of application will. be thereby reduced to the point that the brakes cannot be applied within what are considered safe limits as defined by various state laws. Failure to meet theseminimum requirements by a common carrier would have serious results. Thus the demands made upon brake systems to be effective and also safe under a wide range of conditions are conicting.

The problem is especially acute with passenger buses because of the great dilierence between the net weight of the vehicle alone and the gross weight loaded. This problem is increasing inV modern designs because the weight of the vehicle is now decreasing for a given body size; or differently stated the ratio of the live load to the dead load is being increased by modern designs. As vehicles become lighter in proportion to the load carried, it is more and more necessary to design the brake systems in such a way that too much braking efort is not available when the vehicle is empty but there is still adequate brakingV eiiort when the vehicle is fully loaded. While driverY training programs oier some assistance in solving this problem,` driver training is not apnal answer to interval after the brakes are applied, the brakes are stopping only the vehicle because the passengers are movable with respect to the vehicle and therefore are able to continue on at the same rate of speed because of their inertia. lf time permits, passengers brace themselves by muscular reaction or by grasping a seat, strap or other part of the vehicle. In a fast stop, they may move forward bodily into violent contact with some part of the vehicle. In either case, the passengers thereafter move at the same speed as the vehicle and decelerate at the same rate. After the passengers have reached the same `rate of deceleration as the vehicle body, the brakes are operating on the total mass of the vehicle and the passengers; and it will be appreciated that this mass may be considerably larger than the mass of the vehicle alone. Modern buses may have a live load of more than one third of the total gross weight.

Another complicating factor in the solution of the problem is that the co-ecient of friction on the brake shoe is not constant. It is less at high speeds than at low speeds. At the same time, in reducing the speed of the heavy vehicle from a high value to a low value the heat generated within the brake may be suicient to reduce the co-etcient of friction at the lower speeds to a value considerably lower than the co-ellicient existing if braking were started at the lower speed. An attempt to increase the power of the brakes to provide a reserve against this contingency results in the danger of locking the wheels when the brakes are applied at a relatively high speed, with obviously dangerous results.

Thus it becomes a general object of my invention to control the retardation of a vehicle, resulting from appli,- cation of the vehicle brakes,- in such a manner that the deceleration is decreased immediately after reaching a predetermined deceleration and is then restored to' the higher predetermined value after lapse of a sufficient interval of time that the speeds of the passengers and the vehicle are equalized.

It is similarly an object of my invention to control the retardation of the vehicle in such a manner that the rate of restoration of the predetermined deceleration may be determined, at least in part, by the load of the vehicle in order to adapt the braking effort automatically to the load;

Another object of the invention is to reduce the maximum deceleration possible at low speeds in a manner to lcompensate for the greater co-ecient of friction existing when the brakes are initially applied at low speeds in lorder to protect the passengers against discomfort or injury caused by the greater percentage change of speed possible when the brakes are applied at low speeds` A An object is to safely raise the maximum iluid pressure or increase the lever ratio at the brake assembly.l

Another object of the invention is to make the maximum brake effort and deceleration available at highV speeds in order to compensate for the greater momentum of the vehicle and its load, in a manner consistent with the above objects.

The above and other objects of my invention are f achieved in a control system constructed according to systems. Suitable means is provided in the brake system which may be operated automatically to limit the pressure at Ywhich fluid is supp 'ed to the brakes to apply them. In an air brake system, this pressure-.limiting means'i's an exhaust valve connected to the air line between the sou-refe of compressed air and the brakes, which valve can be opened to .regulate or limit the' maximum airY pressure in the line and therefore the maximum'force-'with which the brakes are applied. This valve is normally closed and remains so during slow stops or the like requiring low.

pressure to apply the brakes; but in arpreferred form of my invention'the valve can be ope'nedbyV either one .ur

both of two valve operating means when a limit on fluid f' pressurei'sdesiredfj, n.

1 The iirst of these valve operating means isV responsive to deceleration of the vehicle and opens the exhaust valve to a greater extent with an increasing rate of deceleration. TheV other valve operating means is electrically actuated independently of the -iirst means and is adapted to override .Y the rst means tothe extent that. it may cause a greater degree of opening of the exhaust valve than the opening produced by the deceleration responsive means. This electrically Yactuated 'valve` operating meansV is controlled byV anV electricY circuit which includes a iirst pressurejresponsive switchV that energizes theY circuit initially andV closes a second' switch which is in series with the electrically actuated valve operating means to open the exhaust valve under certain conditions. The circuit also includes a time delay circuit which after a suitable is provided with an annular groove 28.V An O-ring or similar packingmernber at 27u provides a'nair tight seal around ,the valve-member to prevent. the escape of air from'the'open end of chamber 26.' Y

Conduitl23 communicates Vwithinlet passage 29 which opens into chamber 26,. Exhaust passage communicates between chamber 26 and the atmosphere. Passage 30 is preferably narrowY and elongated in the direction of movement of valve 27, as shown diagrammatically in Fig. 1, in order to change the area of the passage open for eiiast'of'airfapproximatelyin proportion to 'the movement 'of the valve 2.7.Y Y `Valve member 27V is mountedon rod 32 `which passes entirely through'valve body 25 and extends beyond the body' at oppositeends thereof. When in the rest position shown, valve member 27 Vrcloses exhaust port 30 Yand preferably also inlet port 29. When the valveV fmembcnis moved to the left as viewed in Fig.r3,ygroove V28 is placed inregistration with 'passages 29 and 30, allowing air vto iiow'from conduit 23 through inlet passage 29 and groove 28 to exhaust passage 30.Y The rate of escape. is deterf Vmined by the net area of exhaust passage 30 uncovered by vpiston 27; and this in turnis controlled `by the amount of movement of pistonr27 togthe left. 'Y -Valve member 27 is Vnormally biased to the closed position by spring 34 Vwhich bears at one end against -ring attained will be rmore readily'understood by, reference to the following description and to the annexed drawing, in which: Y.

Figi is a schematic representation of a compressed air f brake system for a vehicle showing the application thereto offa preferred form of control apparatus foi-'regulating .the retardation of the vehicle;Y

lig. 2. is a schematic diagramV of the' electrical circuit of. the improved. control apparatus; andY AFig.V 3 is, a longitudlnaly median section through the exhaust valve for the brake air line and the solenoid for operating the valve.

"Referring now to Fig. l there isV shown schematically a Vconventional type of brake system operated by compressedV and including a reservoir or storagetank 10 as a source of operating an under pressure. Air taken Y from the tank is replenished by Ta compressor unitfin a ltnovvnY manner, such compressor unitnot being shown m' the drawing. Reservoir 10 is connected by line 11 to brake valve 12 which'is here shown as being'a foot oper ated type.Y of valve by which' the` 'vehicle dpivereffects application of the'brakes. 'd x Yp Y Y vLWhen open, air passer through brakevalvelZV and line 14into distribution line 15..V Line 15 is connected at one` end VtoVV branch lines; 16, each'of which"Y leads VV'to a diaphragm type motor Vunit "18, Vthatfapplies powerA to a connected to a leverV ar'rnfnot shown inthe Adrawings, ofv afront wheel brake assembly.V 'Distributioifline 1 53-.is also connected Vto branch'lie's L2li-'whichV Yapply/air toesimilar motor units 18 operating rods 1,9 fwhich similarly connected by arms, Vnot shown; to'V thebrak'es onf/theV rear wheels "llhese parts of the brakesystems ae 'illustrative of many well'knownlbrke installations 'Y .aridneednot bedescribed in'detail.- "5 i' l 'Anelement of my control. apparatus -is suitable means forh Vtha-pressureat' whichtheoperating -uid-in theV bfkeSYSfemf @en be'supplidzt@ th Brakes fr'appl-ctionlofQth't',V brakes'.- 'Ina1r compressed airbrake systeiinj 4able core. i A fixed core 31 limits travel of the Yvalve rod.

Movement ofthe valve member 27 to an open position can be effected by pendulum 40 that is pivotally mounted at 41 to 'an arm'formingV a portion of housing 24. Valve 22 and the pendulum are mountedV onV the vehicle in such a Way' that the pendulum is at the rear end of` the valve Withj respect tothe directionY of forward movement `of the vehicle so that when the vehicle slows down the pendulum moves relative to the valve in the direction of arrow 42. The pendulum bears Vagainst the right handV end of valve Vod 32 so that forward movement of the pendulum 40 moves the valve rod and valve 27 to bring groove 28 opposit'ev exhaust Vport 30', permitting the Yescape of air through thisfport from distribution lines- 15.,V Pendulum 40 is held'normally in a neutral position with its suspension rod vertical by tension spring44 which is attached atonel end to a` xed portion of Vthe vehicle frame and at the other lend to thependulum. Spring 44 is typical of meansthat'may be used torestrain the pendulum to damp out small shifts inpsition'in response tothe small changes in vehicle speed. Also, it may be sti-'ong enough ttiv influence the action ofthe pendulum in a desired manner.Y YFor example, the spring assists in the return of the pendulumV to the neutralposition. allowing: valve Y27' to 'closet It may-be advantageous to'add a dash-pot device as indicated diagrammaticallyV Vatv 43V for the purpose of damping the'pen'dulum motion in either for both directions. Such devices'are Well'known. 'Y

` Valve 27 can also, be opened' by means of solenoid 36.

When lthe v'coil of the 'solenoid is energized, it"Y drawsV valve rod 32 toward the left, until it is stopped by engagement Vcan `movey away :fromY thev pendulum., BecauseY of this independent-operation offthe-'two separatev valve operatingmeans, the electric means can override or exert a superior inuence upon valve 22 in that the electric means can move the valve piston 27 regardless of any inuence exerted on the valve by the pendulum.

Enclosed within the dotted rectangle 45 of Fig. l is an electric circuit for controlling the energization of solenoid 36 and thereby controlling operation of the uid pressure limiting valve 22. This circuit is shown sche matically in detail in Fig. 2.

The control circuit includes battery 48 as a source of power. This may be the ordinary six -volt or twelve volt battery commonly installed in the vehicle to operate the ignition system and other auxiliaries. The battery is connected to a ground 49 which may be the frame of the Vehicle providing a common return line. To each terminal of the battery there is connected one of the two leads 50 and 51. Across the two leads there is connected a resistance 52 and a condenser 53 in series with each other. At a point between the condenser and the resistance is connected one terminal of switch 54, the switch being in series with operating coil 55 of a relay. The other side of coil 55 is connected to lead 50. Switch 54 is a pressureoperated type, as for example the well known type of switch used to operate vehicle stoplights. It is designed to respond to an increase in pressure in the brake system by. closing the contacts at a relatively low pressure, as for example 4 p.s.i. The switch 54 is connected to air distribution line by branch conduit 56.

Relay coil 55 actuates normally open switch 57 which is closed when coil 55 is energized. One side of switch 57 is connected to lead 51 and the other side is connected to lead 58 which connects Switch 57 to the common center contact of a double throw switch 60. The normally closed contact 61 of switch 60 is not used, and the normally open contact 62 of switch 60 s connected to one side of solenoid 36. The other side of solenoid 36 is connected to lead 50. Thus switch 57 is in series with switch 60 and solenoid 36.

A second normally open switch 64 is connected between leads 51 and 58 in parallel with switch 57. This switch 64 is part of a relay having an operating coil 65 which is connected at one side to lead 50 and at the other side, by lead 66, to the normally open contact 67 of switch 68. Switch 68 -is a double throw switch of which the common center contact is connected by lead 70 through condenser 72 to lead 50.- The normally closed contact 71 of switch 68 is connected directly to lead 58.

Ihe two switches 60 and 68 may be entirely separate switches; but they may conveniently be the two poles of a double-pole, double-throw. switch in a single relay in which both poles are operated by relay coil 74. Relay coil 74 is connected at one side to lead 50 and, at the 4other side through lead 75 to a pressure actuated switch indicated generally at 76. The latter switch is indicated in its entirety by the dotted rectangle.

SwitchY 76 is any suitable type of pressure sensitive electric switch which is connected to the brake system, preferably to line '15, by branch air conduit 78. One well known type of switch as at 76 is that manufactured by Melletron Corp. of Los Angeles, California, designated Model 424. Switch 76 comprises a pair of single pole switches 80 and 81 which are normally open but are closed individually by a pressure responsive member 79, as for'example adiaphragm or a similar member, moved in response to au increase in air pressure transmitted to member 79 through branch conduit 78. Switch 80 closes at a lower pressure than does switch 81; and each switch is preferably opened by a pressure drop of approximately 10 p.s.i. below the air pressure at which they close. For example, but without limiting the invention to any particular values, switch 80 may be set -to close at 60 p.s.i. and then open when pressure drops to 50 p.s.i. while switch 81 may be set to close at 70 p.s.i and reopens when pressure drops to 60 p.s.i.

Lead'75 is connected to one side of both switches 80 and 81. Switch 81 has its other side connected directly to lead 58 while the other side of switch 80 is connected to lead 58 through a normally closed, speed responsive switch 84 which is adapted' to open at a predetermined vehicle speed, for example 2O miles per hour. Switch 84 may be any suitable known type of switch, switches of this character already being known and used onvehicles for various purposes.

As an optional feature in order to introduce certain time delay characteristics which will be more fully disclosed in the operation of relay coil 65, there may be placed in parallel with coil 65 a variable resistance 86 and a ixed resistance 87, the two resistances being in series with each other and connected to leads 50 and 66 across coil 65. l

Another optional feature is the provision of lead 89 which connects one contact of switch 54 through normally open switch 90 to lead 50. Switch 90 is Iarranged to be actuated by coil 65 in unison with switch 64. When energized, coil 65 closes switch 90 and provides a shunt circuit around coil 55 that discharges condenser 53, for reasons that will be explained.

When installing the circuit on a vehicle, the solenoid 36 is advantageously placed in housing 24 to be located at the exhaust valve 22, for obvious reasons. The pressure responsive switches 54 and 76 may be located in any position on the vehicle desired in order to best regulate operation of the brakes. Preferably branch conduits 56 and 78 leading to the switches 5 4 and 76 are short in order to minimize the time required for build up in the switches of iiuid pressure `to operative values. Speed responsive switch 84 may have its location ondthe vehicle dictated by mechanical requirements. The time characteristics of the entire control means can be regulated to some extent by the relative position of the various elements. For example, the closer branch conduit 78 is to exhaust valve 22, the quicker the response of valve 76 to a decrease in air pressure caused by exhaust from valve y22. However the other elements of the circuit, such as the switches, relays, resistors, condensers, and the like, which are all electrical components, can be grouped in a single cabinet which grouping presents obvious advantages in ease of wiring, accessibility for service, and the like. Such a cabinetvis indicated diagrammatically in Fig. 1 by the rectangle 91 in order to complete the schematic illustration of my invention.

Having described the apparatus and its control circuit, I shall now describe the operation of my invention. When the brakes .are released, the apparatus for controlling the application of the brakes is in a rest or neutral condition with the circuit elements in positions shown in Fig. 2. All of the switches are open, except switch 84 which is normally closed. The operationof the circuit varies with diierent vehicle speeds at the time of applying the 4brakes and with the force with which the brakes are applied. The diierent degrees of control exerted on the `brake system will be described with reference to several typical cases.

Assume now, as case #1, that at a relatively low speed, say 15 miles per hour, the brakes are'applied With great force to produce rapid deceleration. Fluid pressure in line 1S is transmitted through branch line 56- to close switch 54. This occurs almost immediately as switch 54 is designed to operate at a selected low pressure, say 4 p.s.i. or higher. When switch 54 is closed, coil 55 is `energized and closes switch 57 establishing a circuitv from battery 48 and lead 51 through switch 57 and lead 58 to the common contact of switch 60. The normally open contact 62 of switch 60 is still open, leaving solenoid 36 de-energized. Closing switch 57 activates the circuit as a whole so that it can respond to other conditions. This initial energization takes place during the initial stage of every application of the brakes.

Because the driveris attempting Ato bring the vehicle established through switch brakes.V

Y to a rapid stop he opens throttle valve 12 Wide, and the pressure lin line quicklybuilds up to 60 p.s.i. or-more. It is-assumedfor illustration that 60 p.s.i.-is the pressure,

transmitted throughbranch line 78 to pressure'sensitve switch 76, at Vwhich switch 80` closes. Switch 81 ordi- Y From lead 58` a circuit is now 84, which remains closed, switch 80, lead 75, torelay coil 74 and lead 50. The resulting energization ofjcoil 74 changes the position of the double throw switches y60 and68, the switch blades now being moved intoV engageme t with the normally open contacts-@fand 67 respectively.4

When switch 60 closes, the circuit energizing solenoid 36 is completed; and upon energizatio'n solenoid'36 moves narily remains yopen'.

' 'valve rod 32 toward the left in Fig. 3 to uncover exhaust port 30. This allows operatinguid under pressure to escape from distribution line 15 and has the effect of Y limiting to 60 p.s.i or slightly more the fluid pressure Vwhich canbe supplied to brake motors 18 for the appli- 1 cation of Ythe-brakes..

This escape of `fiuidhasthe ultimate effect'of limiting the retarding force which Vcan be applied to stop the vehicle. Y

It maybe, assumed under Athe'cohditions of case #l that the operation of pressureY sensitive switch 76 takes place 'within' three seconds'following application of the The control circuit includes Va rst time delay means Kfor'deei1ergizi1`ig the entire circuitY after a predetermined Vinterval of time,'which for purposes of ,this

illustration isra'ssurned to be three seconds, which is'ample time vfor pendulum 49. to move Vinto control of Vvalve-,27. Capacitor '53 fand resistance 52,Y are so arranged as jtoV formV a kresistarice-capacitance circuit which limits the length of ,timefthat Yrelay coil 55 remains energized. The valuesA of the resistor` and capacitor are selected so that following a predetermined time interval, here assumed asrthreeseconds but it may be more or less, after Y'switch V54- is closed,v the currentY fiowing to relay 'coil 55 drops below the value required to maintain switch 57 closed and as a result switch 57 opens. Since switch 57 is in serieswith switch 60 and Ysolenoid 36, the opening of this switch de-energizes solenoid 36, releasing valve rod 32. Consequently, the circuit energizes solenoid 36 for a maximum of three seconds, or whatever period the characteristics of the resistance-capacitance circuit determine. After this period solenoid 36 no Ylongernoperates the pressure. limiting valve 22j. it may be noted here that, as explained below, the solenoid 36 may bede-energized in less than three seconds. Y

. When solenoid 36 is de-energized, thevalveV rod 32 is solely under VYpositional Vcontrol of pendulum 40; and the A position'of the pendulum is determined at any instant by The displacement of the ant of' case #L if thel driver releases the brakeswithin the three second period; when fluid pressure Vdrops below 50 p.s.i. switch Sllrisl opened". Opening this switch deenergizes relay`coil 74 and opens switches 60 andV 68.

The result is that, in less than three seconds frominitial` application of the brakes, Athe electric means for operating the pressure-limiting valve 22 relinqushes control in favor of-the control exerted by pendulum` 4i) Vin responsetodeceleration ofthe vehicle. Y When switch 57 closes andswitch 68 isiirrthe rest position shown, with the normally closedy contact 71 engaged,Vv condenser 72.5.5. connected .byrl'ead 70; between leads 50 and Y58 and throughiswitch 57 Yto lead' 51, thus *After said interval of Y charging the condenser. When the movable Vswitchblade Y 72 in combination with resistances'SG andY SI-forms a If lit Y closesy this Vaction has no `eilect on the circuit. As avarisecond independentV resistance-capacitance timing circuit which can be Vadjusted Vto determine the length of time coil 65 holds switch 64 closed. Y

When switch 90 closes, it immediately discharges condenser 53, openingV switch 57 in les's'than the threeseconds established by the Viirst'timing circuit. With switch 57 open, solenoidr3`6 is now energized through switch 6,1 and is dezenergized when either switch 64 or 80 opensl 'the last4 mentioned timing circuit can now break the circuit energizing solenoid 36 before pressure actuated switch'76 cani do iso. Adjustment of the timereharacteristie of this circuit can be obtained by making resistor 86 variable, while therange of adjustment can be limited by placing xed resistance 87 in series with variable resistance r86. By the use ofV thisV independent timing circuitthe time that solenoid 36 remains energized after switch` 60 is closed is now controlled by either o 'two conditions, one of thern'is a'drop in uid pressure api plied to the brakes and the other is'the passage of time. 'y This second timing'circuit becomes operative after closing switch 164. The auxiliary circuit through switch 9i)V is preferably used to neutralize the tirst timing circuit and Vdeiinitelytransfer timecontrol to the second` circuit. It also operatestoprevent a secondrpulse lfrom energizing solenoid 36 during` a single application ofthe brakes. .In Case #2s assume thatfrgm 'any Speed' the drivsf bringsthe; vehicleto aslow stop. The passengers are decelerating at a rate; close to that oi thevehicleandinow the regulation problem is. simpler.; `Whenmaking a slow stop thedriver only opens valve 12 partial1y,"andtlie fluid pressure in line 15 and,otherpartsaofthebrake systems remainstwithin a relatively;V low range of values. 1n this lowrange of values, switch.57, closes asY described but switch 76 is not actuated- Accordinglyv switch 60 remains open `and'solenoid 36 isnotgenergized. ThereafterV the same interval ofrtime to reopenv switch 57 but opening this switch now has no e'ffect upon solenoid 36 since Vthe electrically actuated means did not operate the exhaust valve.

1 The pendulum can exert adequate ,control over the pressure limiting valve 22 whenthe stop, is a slow one. The, pendulumexerts its greatest eiect when the bus is empty. Then the need for high Yfluid pressure is least. During slow Yd{acelera-tion` the' pendulum. operates satisfactorily. kDuring fast deceleration the response of the pendulum is .too slow and the electrically actuated means for operating valve 22 provides a faster pulse type respouse when brakes are applied. After initial control Ais exerted and passengers and vehicle areslowing at the .same rate, control can be turned over to the pendulum.

YSwitch 84 is adjusted toV open in response to the speed of the vehicle atV some value, for example 20 m./p'./h., which is the boundary between Vthe high and low speed ranges. In case #3, assume the vehicle is moving faster than 20 m./p./h. when'th'e brakes` are applied. VWith switch 84 open,the operation of the circuit-is exactly the same as previously describedV except that ,switch 80 `is ineective to energize coil 74 and thercircuitf previously completed-by closing switch 8,0 isY nowcompleted by closing switch 81 in parallel withswitch $0. Switch 81 closes ata higher-fluid pressure line,15,.for example 70 p.s.i., andltke switch soit opens after a' drop of say ten pounds 9 in the lluid pressure,reopen ing at 60 psi. Thus the circuit responds to a higher vehicle speed by operating the valve 22 in a manner to raise the `limit that the open valve places on unit pressure of the brake operating uid when the vehicle speed exceeds a predetermined value.

VIt can thus be seen that by limiting the maximum deceleration of the vehicle near the time Vof vehicle load shift, the vehicle wheels are not apt to lock which means that higher maximum air pressure may be used in the brake system with safety. The load shift occurs when the passengers and vehicle are decelerating at the same rate or when the dead weight of an empty bus has transferred the maximum load to the front wheels.

It will be clear from the above that Yvarious changes may' be made in the design and arrangement of the various elements of the apparatusfor regulating the retardation of a vehicle without departing from the broad spirit and scope of my invention.V For example, the pendulum may be omitted, and only the electrical circuit means used to control the pressure-limiting means. Also, this pressure-limiting means may take other forms and be at other locations. It can be a known type of means in brake valve 12 or line 14 to limit the passage of operating fluid and thereby limit the pressure supplied to apply the brakes. In a hydraulic system with a vacuum booster, valve 22 can be so located as to limit the vacuum developed and thereby the assisting force developed to apply the brakes. Accordingly it is to be understood that the foregoing description is considered to be illustrative of rather than limitative upon the invention as defined in the appended claims.

I claim:

l. In a iiuid actuated brake system for an automobile, apparatus for regulating the retardation of the vehicle by application of the brakes, comprising: a normally closed valve venting the system to the atmosphere to limit the pressure in said system of fluid supplied to the brakes; electrically actuated operating means for opening the normally closed valve; and electrical circuit means for energizing said operating means, said circuit means including a pressure-responsive switch subjected to iiuid pressure existing in said system to be actuated when the pressure in said system exceeds a predetermined brake operating pressure, said switch energizing the circuit means to open the normally closed valve to limit pressure in the system to said predetermined value, said electrical circuit including circuit elements that tie-energize the electrical circuit after a predetermined interval of time following application of the brakes to render the circuit means ineffective to open the normally closed valve.

2. In a uid actuated brake system for an automobile, apparatus for regulating the retardation of the Vehicle by application of the brakes, comprising: pressure limiting means operable to limit the pressure at which liuid is supplied to the brakes; electrically actuated operating means for operating said pressure limiting means; and electric circuit means controlling said electrically actuated operating means and including a first fluid pressure actuated switch adapted to close in response to a low uid pressure in the brake system and energize the circuit means; relay means having an operating coil energized by closing the first switch, and contacts closed by said coil when energized; a second switch connected in series with the relay contacts and with the electrically actuated operating means; and pressure responsive means closing the second switch in response to a relatively higher predetermined uid pressure in the brake system, thereby energizing the electrically actuated operating means when the relay contacts are also closed.

3. The combination as in claim 2 in which the means closing the second switch also opens the second switch in response to a predetermined drop in uid pressure; and which also includes time delay means to open the second switch after the lapse of a predetermined length of time following closing the switch.

4. In a fluid actuated brake system for a vehicle, ap-,f paratus for regulating the retardation of thevehicle effected by application of the brakes comprising: fluid pres- Sure limiting means operable to limit the pressure at which uridis Supplied to the brakes; electrically actuated operating means for operating said pressure limiting means; and electric circuit means controllingsaid electrically actuated operating means and lincluding a first switch; fluid pressure actuated means adapted to close the iirst switch in response to a low uid pressure in the brake system; relay means including a second normally open switch connected in series with the iirst switch and in series with the electrically actuated operating means; means to actuate the relay means and close the second switch in responseto any one of a plurality of higher pressures in the brake SYStem to energize the electrically actuated operating means; and means responsive to the vehicle speed to render ineffective the response of said relay actuating means to all but one of said higher pressures.

5. In a -iluid actuated brake system for a vehicle, apparatus for regulating the retardation of the vehicle effected by application of the brakes comprising: fluid pressure limiting means operable to limit the pressure at which liuid is supplied to the brakes; electrically actuated operating means for operating said pressure limiting means; and electric circuit means controlling said electrically actuated operating means and including a iirst fluid pressure actuated switch adapted to close in response to a low fluid pressure in the brake system; relay means including an operating coil in series with the first switch and a second normally open switch connected in parallel with the first switch and in series with the electrically actuated operating means; a pair of switches connected in parallel and closed sequentially oy means responsive to fluid pressure in the brake system to energize the electrically actuated operating means; and normally closed switch means in series with the one of said pair of switches that closes first adapted to neutralize said one switch when the vehicle speed exceeds a predetermined value.

6. In a iiuid actuated lbrake system for a vehicle, apparatus for regulating the retardation of the vehicle effected by application of the brakes comprising: uid pressure limiting means operable to limit the pressure at which fluid is supplied to the brakes; electrically actuated operating means for operating said pressure limiting means; and electric circuit means controlling said electrically actuated operating means and including a iirst switch uid pressure actuated means adapted to close the first switch in response to a low uid pressure in the -brake system; time delay means opening said first switch after a predetermined time; a second normally open switch in series with the first switch and with the electrically actuated operating means; a third normally open switch in parallel with the first switch and in series with the second switch; means to close both the second and third switches in response to a relatively high uid pressure in the brake system to thereby energize the electrically actuated operating means; and a second separate time-delay means opening said third switch after a predetermined time.

7. The combination as in claim 6 that also includes an auxiliary circuit including a fourth switch closed simultaneously with the third switch to cause the iirst time delay means to open the rst switch substantially im' mediately.

8. In a uid actuated brake system for a vehicle, apparatus for regulating the retardation of the vehicle eiected by application of the brakes comprising: a normally closed valve venting to the atmosphere for limiting the pressure at which iiuid is supplied to the brakes for application of the brakes; a first means for directly operating said pressure-limiting valve in response to deceleration of the vehicle; separate electrically actuated operating means for simultaneously operating said Y' pressure-limiting means independently'of said vrst operating means; and electric circuit meansV controllingy said electrically' actuated `operating means andgincluding pressure-responsive switch means closed by a high predetermined uid pressure in the system to actuate the operating means andthereby limit uid pressure'inthe system substantially Vto 'said high pressure. 5 9. The combination asin claim 8 in which the circuit means includes time delay means for cie-energizing the I electrical' circuit'after a predetermined intervalof time following application of the'brakes'to render ineffective 'the velectrically actuatedoperating means, to'restore the pressure-*limiting means to the sole control'of the 'first operating means.

10. 'In ValuidV actuated brake system" for a vehicle', apparatus for regulating the retardation of the vehicle effected by Yapplication of the brakes comprising: uid pressure-limiting'means operable tolimit thefpress'ure at which udis supplied to the brakes; electrically actuated operating means for operating said pressure'limiting i2' Y means; and electric circuit ,means'controllingY said-elec` trically actuated operating means and including a pair of switches connected in parallel Ywith Yeach other andin' serieswith the electrically actuated operating means and' Y AReferences- Cited in the Vtile of this patent uNlrnnsrAras PATENTS Y* 992,183 e oct. v27, 190s Turner et al.' 2,096,505 VBaughman Oct.V 19,- V1937 2,109,168 YMiller Feb. 22; 1938 2,198,029V Farmer Apr. 23, 1940 2,256,285 Y Hines Sept.' 16, 1941 2,396,424 Hines et Yal.V Mar. '12, 1946 

